Synthetic polymers (also referred to hereinafter as “resins”), such as styrene resins, epoxy resins, silicone resins, polyurethane resins, phenolic resins, urea resins, polycarbonate resins, polyester resins, polyethylene resins, polypropylene resins, polyamide resins, polyimide resins, polyvinyl alcohol resins, styrene-butadiene-styrene resins, acrylonitrile-styrene resins, ethylene propylene rubber, acrylonitrile-butadiene-styrene resins, vinylidene chloride-acrylonitrile resins, vinylidene chloride-vinyl chloride resins, and ethylene-vinyl acetate resins, have already found use in various applications. Materials having the properties of inorganic compounds, such as high strength, high elastic modulus, heat resistance, and electric properties, while retaining the properties of polymers, such as flexibility, low specific gravity, and formability, are under active development as means for improving the various properties of resins, such as mechanical properties, heat resistance, and flame-retardant properties. So-called polymer nanocomposites—i.e., composite materials employing inorganic particles wherein at least one of the three dimensions thereof is in the nanoscale range—are receiving attention as such means for improving the resin properties, replacing conventional resins reinforced with fiberglass, talc, etc.
Examples of such conventionally-used inorganic compounds include clay minerals, such as layered inorganic compounds. Clay minerals, however, have poor dispersibility to polymers. To tackle this problem, Patent Document 1, for example, proposes a method of displacing alkali metal ions contained in a layered inorganic compound with organic onium ions and making the compound organic, to facilitate the dispersion of the compound into polymers and improve the polymers' mechanical properties.
The method of Patent Document 1, however, is still not at a satisfactory level, and there still have been demands for methods of producing polymer nanocomposites capable of improving both mechanical properties and heat resistance of the polymer matrix at the same time.
Against this backdrop, studies are being conducted on a technique of starting radical polymerization in a monomer from the interlayer surface of a layered inorganic compound (called “surface-initiated radical polymerization”) to cause interlayer exfoliation and improve the dispersibility of the layered inorganic compound to resins. Non-Patent Documents 1 to 3 disclose methods of synthesizing polystyrene nanocomposites using montmorillonite modified with organic ammonium salts having the ability of initiating living free radical polymerization. The disclosed nanocomposites, however, have drawbacks in that the organic ammonium salts have poor heat resistance and that it takes a long time for the polymerization.
The same goes for azo-based polymerization initiators: all known methods for immobilizing azo-based polymerization initiators onto the interlayer surface of layered inorganic compounds also involve introduction of an onium salt, such as an ammonium salt, into the azo-based polymerization initiator's molecule and immobilization thereof by utilizing electrostatic effects, thus giving rise to the above-described drawbacks regarding heat resistance etc. In addition, because the molecular weight distribution is not controlled as contrasted with living free radical polymerization initiators (Non-Patent Documents 1 and 2), the molecular weight distribution tends to become broad. Controlled molecular weight distribution intimately relates to the exfoliation and dispersion of layered inorganic compounds: a narrow molecular weight distribution means that the polymer will grow uniformly from the interlayer surface, which will contribute to the exfoliation and dispersion of the layered inorganic compound. In other words, a narrow molecular weight distribution benefits the exfoliation and dispersion of layered inorganic compounds, whereas polymers with a broad molecular weight distribution without controlled polymer growth tend to result in incomplete exfoliation and dispersion of layered inorganic compounds.